This invention relates to improved lightweight grids for lead-acid storage batteries. The invention relates more particularly to laminated battery grids having an open mesh portion for supporting battery paste which portion is made of a relatively thin layer of lead foil coextensive with and adhesively bonded to a lightweight supporting layer of plastic.
A typical 12-volt automotive battery has an inert plastic or rubber case which contains six voltaic cells and employs a sulfuric acid electrolyte. Each cell is made up of positive and negative plate groups interlaced with each other so that the positive and negative plates alternate. Electrically nonconductive, microporous separators are positioned between plates to electrically isolate them while allowing the free passage of electrolyte.
Each plate consists of an electrically conductive metal grid, usually made of lead or lead alloy, spread with a layer of "leady oxide" paste. By the term "leady oxide paste" herein is meant a spreadable combination of lead oxide, lead sulfate, sulfuric acid, water and other additives conventionally used in making chemically active lead acid storage battery materials. When battery grids are pasted with such material, immersed in a sulfuric acid electrolyte, and thereafter subjected to a source of direct current, the leady oxide paste on plates connected to the positive electrode is substantially transformed to lead dioxide. Paste on plates connected to the negative electrode is substantially transformed to sponge lead. This process is referred to herein as "forming" or "charging" the battery. The resultant lead dioxide and sponge lead materials are referred to herein as chemically active battery pastes or materials. A battery grid after it has been pasted and formed is referred to as a battery plate. Lead dioxide is the chemically active material of positive plates and sponge lead the active material of negative plates. Both active materials react with the sulfuric acid electrolyte to form lead sulfate and produce an electric current when an electrical circuit is made between positive and negative terminals.
Conventional grids are made entirely of lead. They are characterized by an open mesh portion for supporting the active paste materials and a header portion consisting of a top bar for collecting grid current and a tab or lug for joining plates of like polarity together in a cell. By lead herein is meant substantially pure lead metal as well as alloys of lead of the type used in lead acid battery components. Lead is a preferred grid material because it is resistant to corrosion in the battery environment, it is easy to form into a desired grid shape, and it is a good electrical conductor. However, pure lead is so soft that it must be alloyed with hardening elements such as antimony or calcium to form a self-supporting grid strong enough to withstand economically advantageous automated grid pasting and cell assembly operations. Alloying lead adds to battery manufacturing cost, and the hardening elements in alloyed lead may interfere with battery life or performance. To meet strength requirements, a substantial amount of lead metal is required for each grid. Thus, battery grids made entirely of lead are relatively expensive and contribute substantially to battery weight.
The present invention relates to improved battery grids wherein a substantial portion of the lead is replaced with a relatively inexpensive lightweight polymeric plastic.
U.S. Pat. No. 3,956,012 to Scholle shows a composite battery grid consisting of a molded plastic grid in which a strand of lead wire is embedded. Such grids are difficult and expensive to manufacture, at least in part because plastic grid portions must be individually formed by injection molding or other suitable process. Moreover, means must be provided for manipulating and shaping the lead wire portion for each grid and then embedding it in the plastic portion. Thus, such grids cannot be made and assembled into plates by continuous automated processes like those currently used to manufacture all lead grids and form them into plates. Furthermore, the Scholle grids are generally thicker than all-lead grids because the lead wire portion must protrude from the plastic portion enough to provide electrical contact between itself and the active battery material.
Other prior art composite grids have been made by first molding a plastic grid shape and thereafter coating a surface with a layer of lead, usually by metal spraying. This method is also expensive because the plastic grids are individually formed and lead is wasted because of metal overspray. Other grids have been made by first spraying a sheet of plastic with lead and thereafter punching grid shapes from the sheet stock. This method produces an undesirable amount of composite scrap, wasting both lead and plastic. Moreover, both processes require the use of metal coating equipment not generally used in the battery-making art.